1. Field of the Invention
The present invention relates generally to magnetic resonance imaging methods and apparatus, and more particularly to a resonance imaging method and apparatus fit for fast object-imaging.
2. Description of the Prior Art
Magnetic resonance imaging technology is applied to obtaining sectional images of objects by utilizing a nuclear magnetic resonance phenomenon and is well known as what is useful for examining the human body.
In order to increase throughput and to lighten the hardship imposed on patients while they are put under control for hours, there increasingly develops a demand for fast imaging methods in this field of technology.
The Magnetic Resonance in Medicine 3, 823-833 (1986) carries an article entitled "RARE Imaging: A Fast Imaging Method for Clinical MR" by J. Henning, A. Nauerth, and H. Friedburg. Under the Fast Imaging Method called RARE, a number of spin echo signals resulting from repeatedly applying a 180-degree radio frequency pulse after spins are excited with a 90-degree radio frequency pulse are differently phase-encoded, whereby one image is reconstructed. Typically, 128 or 256 spin echoes are employed to cause excitation once, that is, to reconstruct one image at one shot. Accordingly, the time required for imaging is reduced to 1/n (n: the number of echoes for use) in comparison with what is required in any ordinary spin echo method in which the excitation is needed 128 or 256 times to obtain 128 or 256 projections necessary for one image to be reconstructed. Since the number of spin echoes decreases in accordance with transverse relaxation time (T.sub.2 relaxation time), however, the image obtained through this method tends to become significantly different in quality from what is obtained through the ordinary spin echo method.
The so-called multishot RARE method has been proposed to make the aforesaid problem less serious. This method is characterized in that the number of spin echoes is set not equal to the number of projections typical of the RARE method as noted previously but slightly smaller than the number of projections; namely, set at 4-16. Under this method, an image similar to what is obtainable through the ordinary spin echo method is obtained to the extent that the number of spin echoes is decreased on the one hand, while it is needed to repeat the excitation 16-64 times when n is 4-16 on the assumption that the number of projections is, for instance, 256 since the number of times the excitation is repeated is given by dividing the number of projections by the number of spin echoes on the other. Therefore, the imaging time is made longer to that extent in comparison with the typical RARE method stated above.